The term ‘internal combustion engine’ shall here include all internal combustion engines, in particular reciprocating piston engines, which can be operated with fossil fuels, such as gasoline, diesel or gas fuels (CNG, LNG).
Increasing the operating temperatures in internal combustion engines with the aim of better utilization of the fuel and the use of additives, such as biofuels, place higher demands on the thermal and chemical stability of surfaces in gasoline, diesel and gas combustion engines. These higher demands can only be met to a limited extent or at great expense by a selection of the compact materials (also referred to as bulk materials) for the engine components.
The terms ‘compact materials’ or ‘bulk materials’ refer to the materials of the substrates to be coated in order to improve the surface properties thereof. They include all materials that can be coated: e.g. steel, Al, Inconel, etc., i.e. materials and engineering materials, from which the components of an internal combustion engine are produced. Among the steels, e.g. 42CrMo4 is a typical material, from which pistons are made.
One method for the cost-effective improvement of the properties of compact materials is the surface treatment thereof, for example by nitriding, physical vapor deposition (PVD) or by spraying coatings of powders or wires using thermal spraying methods (HVOF or plasma). These processes can be used to produce materials that cannot be produced as compact materials, or can only be produced with great effort, but which can drastically influence the wear, friction and corrosion of the compact material surfaces, even if the modification only affects a relatively thin area of the surface.
Normally, the surfaces of the tribological partners in an internal combustion engine consist of different materials or different compact materials, which are optimized for a certain service life in order to reduce the number of service cycles, i.e. the service life is adapted. In such cases, the material or the layer thickness is usually adjusted.
In addition to the desire to reduce wear, reducing the friction losses is another goal in the development of internal combustion engines. Also under this aspect, the targeted modification of the surface plays an important role in the tribological system. In addition to the mechanical properties of the surfaces and their adaptation to one another, the surface materials must also assume important functions, such as the wettability with the oils and a certain control of the chemical reactions of additives and surfaces, without becoming chemically unstable themselves. All these facts show that the optimization of the tribological system of an internal combustion engine is extremely complex and requires flexibility in the selection and coordination of the surfaces. Thin layers offer here a higher degree of flexibility than is the case with compact materials.
EP 1 022 351 B1 describes the coating of the inside of a cylinder, the cylinder running surface, wherein the layer is applied by plasma spraying. The mostly ferrous layer also contains FeO and Fe2O3 proportions, wherein the proportion of bound oxygen is between 1 and 4 wt %- and the Fe2O3 proportion is below 0.2 wt %. The admixture of oxide ceramic powders with a proportion of 5 to 50 wt % to the process gas is recommended as a particular advantage in order to achieve even better coefficients of friction. TiO2, Al2O3-TiO2 and Al2O3-ZrO2 are indicated as oxide ceramic powders.
EP 1 340 834 A2 claims a cylinder running surface layer for reciprocating piston engines, wherein this layer has been applied by a plasma spraying method. The layer produced in this way has a content between 0.5 and 8 wt % of bound oxygen and includes embedded FeO and Fe2O3 crystals. In addition, the layer has a porosity degree between 0.5 and 10% and is honed to a certain roughness. The pores in the layer form microchambers which serve as a reservoir for lubricant and which promote a uniform oil distribution in the tribological system.
WO 2015/128178 A1 describes the surface treatment of a piston ring, which consists of a combination of surface nitriding and PVD coating. The running surface of the piston ring to the cylinder running surface is coated with a CrN layer, preferably deposited by ion plating. All other surfaces are subjected to a nitriding process. The deposition of the PVD layer on the non-nitrided piston ring surface is primarily intended to prevent cracking in the coating.
US 2014/0260 959 A1 claims a coated piston ring made of iron-based material, on the surface of which a wear-resistant layer is deposited which contains Al5Fe2 and which has a high hardness. This document also claims a chemical composition of this layer consisting of 52 to 55 wt % Al and 45 to 48 wt % Fe.
U.S. Pat. No. 7,406,940 B2 describes a tribological system consisting of a piston and a cylinder running surface, wherein the piston also has recesses for piston rings which are in tribological contact with the cylinder liner. In addition, the piston skirt is designed in such a way that the surface thereof is provided with a trench structure to improve the lubricating film. The piston ring and piston skirt are also coated with a DLC layer to improve the sliding properties, above all in conjunction with and adapted to a variety of lubricants.
The examples show that surfaces are coated to reduce the wear thereof and reduce friction losses. A variety of coating materials can be used to achieve this goal. Different processes are employed to apply these materials to the substrate surfaces. Furthermore, the state of the art makes it clear that very different substrate materials have to be coated in order to achieve advantageous surface adjustments of the partners in the tribological contact under the respective conditions.
In some cases, the geometry of the component to be coated also dictates the coating process. This is the case, for example, when cylinder bores have to be coated. For such a coating of the inner part of this component, a spraying method is far more suitable than a PVD method. Even in cases where thicker layers of above 100 μm or even above 500 μm have to be deposited, such methods are far more effective than a PVD coating. The latter in rum has economic advantages for thinner layers in the range up to 10 μm or up to 30 μm if the substrates can be coated in a batch process.
Testing the layer pairings of a special tribological system with regard to wear and friction losses could, of course, best be carried out on the real internal combustion engine. However, these tests are too costly to carry them out for all possible material combinations. In addition, there would also be the risk that unknown material combinations could damage the internal combustion engine, which could result in the destruction of the entire test stand.
The above shows both the variety of possibilities and how difficult it is to make a selection of layers and layer pairings.
The object is therefore to provide an improved tribological system, in particular for internal combustion engines, in which two bodies each form components of an internal combustion engine and two material areas are formed on their surfaces, which touch each other at least in certain areas during operation and form a tribological contact. In particular, there is a need to improve tribological systems consisting of piston ring and cylinder or of cylinder and piston. The object is here also in particular to provide further materials or material combinations from which the material areas designed as layers can be produced.